Death-associated protein 3 (DAP3) was previously isolated in our laboratory as a positive mediator of cell death. It is a 46-kDa protein containing a GTP binding domain that was shown to be essential for the induction of cell death. DAP3 functions downstream of the receptor signaling complex, and its death-promoting effects depend on caspase activity. Recent reports have suggested that DAP3 is localized to the mitochondria, but no functional significance of this localization has been reported so far. Here, we study the sub-cellular localization and cellular function of human DAP3 (hDAP3). We found that hDAP3 is localized to the mitochondria and, in contrast to cytochrome c, is not released to the cytoplasm following several cell death signals. Overexpression of hDAP3 induced dramatic changes in the mitochondrial structure involving increased fragmentation of the mitochondria. Both the mitochondrial localization of hDAP3 and its GTP-binding activity were essential for the fragmentation. The punctiform mitochondrial morphology was similar to that observed upon treatment of HeLa cells with staurosporine. In fact, reduction of endogenous hDAP3 protein by RNA interference partially attenuated staurosporine-induced mitochondrial fission. Thus, hDAP3 is a necessary component in the molecular pathway that culminates in fragmented mitochondria, probably reflecting its involvement in the fission process. These results, for the first time, provide a specific functional role for hDAP3 in mitochondrial maintenance.